Aqueous solution for stripping nickel



3,015,630 AQUEOUS SOLUTION FOR STRIPPING NICKEL Robert E. Thompson, Mason, and Charles R. Allison,

St. Bernard, Ohio, assignors to General Electric Company, a corporation of New York No Drawing. Filed Aug. 24, 1959, Ser. No. 835,426 9 Claims. (Cl. 252-401) This invention relates to stripping solutions and more particularly, to stripping solutions for removing platings of nickel or nickel alloys.

Coatings of nickel are well known as corrosion inhibitors on the surface of articles subject to corrosive atmospheres. The deposition of such a coating through normal production procedures may produce in some situations a faulty coating which must be completely removed so that a new coating of nickel may be applied. Some nickel stripping solutions and compounds have been reported. However, in the case of complex assemblies such as, for example, steel coolant passages in heat exchangers having copper fins silver brazed to the steel, it has been found that prior available compositions for removing a nickel coating have not proven satisfactory. In many cases the stripping solutions pit, etch or otherwise injure the under surfaces of the copper, the silver braze or the steel. Although some of the commercial chemical stripping agents may satisfactorily strip nickel from individual materials, such solutions have been found to be relatively slow acting as well as unsuitable for removing nickel platings from such complex assemblies as copper-silver braze-steel assemblies.

It is a principal object of this invention to provide a class of stripping solutions which will remove defective nickel or nickel alloy coatings from simple as well as complex assemblies in such a manner that the surfaces underlying the nickel or nickel coating are not adversely affected or significantly dissolved.

It is another object of this invention to provide a stripping solution which removes nickel coatings from complex copper-silver brazed-steel assemblies within practical time limits.

It has been discovered that the foregoing and other objects of our invention can be obtained by the use of an aqueous stripping solution comprising nitric acid, sulfuric acid, water and an inhibitor in the form of at least one of the nitrates of silver, magnesium, cobalt, aluminum and nickel.

Initial studies indicated that inorganic acids were best suited as strippers of nickel coatings. Since both nitric and sulfuric acids are highly oxidizing inorganic acids it was considered that these be studied as a basis for a stripping solution.

To provide a suitable composition for stripping nickel from complex assemblies including copper base, silver base and iron base alloys and still avoid the objections to presently available commercial stripping solutions, it was found necessary to inhibit base metal acid attack through a judicious balance of acids and inhibitors while removing the nickel coating at a reasonable rate.

It was found that in solutions of nitric acid, steel is rapidly attacked with the reduction of the nitric acid. The iron in the steel is oxidized apparently to soluble ferrous nitrate, thus to allow the reaction to proceed readily. In concentrated solutions, however, the steel is passivated and suffers little attack due to the formation of an insoluble adherent film on the metal surface.

Copper, on the other hand, is readily attacked by nitric ited States Patent F 3,015,630 Patented Jan. 2, 1962 acid. Unlike steel, it does not become passive at the higher acid concentrations.

Nitric acid, however, was selected as the principal reactant or stripping agent. A series of aqueous solutIons were prepared containing HNO in the range of 10-69% to enable selection of a useful HNO range. (All percentages mentioned herein are percent by weight unless specifically designated otherwise.)

Although it was found that nickel was removed at a suitable rate within the 20-70% HNO range, 35-60% HNO was most attractive. Nevertheless, copper, iron or silver based metals were attacked by the acid at an undesirable rate.

It was found that an addition of as little as 1% H in the presence of silver nitrate showed considerable reduction in iron base material attack rate over the above listed aqueous nitric acid solutions. However, copper and silver were still rapidly attacked. A solution of 38.5% HNO 19.8% H 80 and 2.4% AgNO completely inhibited all action on steel after immersion for one hour at 80 F. and greatly reduced attack on the copper and silver.

By increasing the H 80 to about 31% and reducing the HNO to about 33% in a saturated silver nitrate solution, it was found that attack was suitably inhibited on copper and silver as well as on the iron in a complex copper-silver braze-steel assembly.

In order to establish a useful range for nickel removal from materials based on copper, iron or silver or their combinations in complex assemblies, a large number of solutions were prepared some of which are listed in the following Table I:

TABLE I Ex. HNO H280 Inhibitor Remarks 0.7 AgNOs Fe O.K.; slow attack on Ag and Cu.

D0. Cu and Fe O.K.; slow attack on Ag. Cu, Fe, Ag O.K.

Do. Do. Do.

At this point it was established that a satisfactory stripping solution for steel alone comprised of 20-70% HNO with at least 1% H 50 in the presence of a nitrate inhibitor. However, it was found that at least 20% H 80 was required and preferably 45-50% H 80 should be included along with the nitrate inhibitor to prevent the HNO from attacking copper during the nickel stripping period.

Regarding the combination of steel, copper and silver braze, it was established that a range of 23-28% HNO and 45-50% H SO with a nitrate inhibitor gave superior results.

Since a weak link in a copper-silver braze-steel composite was the silver braze, a series of tests was conducted to study the effective range and type of nitrate inhibitor on the action of nitric acid and to study the degree of cooperation of such inhibitors with sulfuric acid to prevent etching of silver braze. Nitrates of cobalt, magnesium, aluminum and nickel were found to be most effective in addition to silver nitrate shown in Table I. Some of the solutions which were prepared in this series of tests are listed in Table II.

TABLE II Useful aqueous stripping solutions tested on silver braze: 50Ag18Cd-16.5Zn 15.5 Cu. at 80 F. (percent by weight) Corrosion Example HNOa H280 Inhibitor rate (inches per year) 33. 6 31. 7 None 0.822 33.6 31. 7 1.7 AgNOa 0.416 32.0 30.1 5.0 Mg(NOa)z 0.115 32. 30.1 5.0 Ni(NO;)a 0. 072

25. 1 47. 9 None 0. 410 25. 1 47. 3 1.2 AgNO; 0.259 24. 7 46. 5 2.9 Cr(NOa)-2 0. 598 24. 7 46. 5 2.9 Cu( 0:)2--- 0.561 24. 7 46. 5 2.9 KNOa 0. 599 24. 7 46. 5 2.9 Al(NOa):.9H2O 0. 156 24. 7 46. 5 2.9 Mg(N0a)2.6HgO 0.0015 24. 7 46. 5 2.9 Ni(N0:l)z.6H20 005 24. 7 46.5 2.9 Ba(NO) l 0. 369 24. 7 46. 5 2.0 P 1 0.326 24. 7 46. 5 2.9 C 0. 175 24. 0 45.1 5.7 F O. 666

1 Severe pitting.

A method which can be used to make the stripping solution of this invention involves first dissolving the nitrate inhibitor in the water before adding the acids. If aqueous solutions of HNO (for example 70% HNO or of H 50 (for example 95%) are used, allowance should be made for the water they contain.

The acids are then added slowly to the water-nitrate solution to prevent excessive heat formation. The solution is then allowed to cool to the desired operating temperature.

When silver nitrate is used as an inhibitor, a white precipitate of Ag SO may form upon cooling to room temperature. However, this precipitate will not affect the solution or its stripping properties.

The stripping rate of the solution is somewhat dependent upon its temperature. Although increased rates can be obtained at 100 F. over those at 80 F., it is preferred that the temperature be maintained below about 105 F. to avoid base metal pitting under certain instances noted at about 110 F. in some cases.

Although this invention has been described in connection with specific examples, those skilled in the art of electroplating, cleaning and stripping will readily recognize the modifications and variations of which this invention is capable.

What we claim is:

1. An aqueous solution for stripping nickel consisting essentially of in percent by weight 20-70 HNO at least 1 H at least 0.5 of a metal nitrate, the metal of said metal nitrate being selected from the group consisting of silver, magnesium, cobalt, aluminum and nickel; with the balance water.

2. An aqueous solution for stripping nickel consisting essentially of in percent by weight 20-70 HNO 20-50 H SO 0.5-6 of a metal nitrate, the metal of said metal nitrate being selected from the group consisting of silver, magnesium, cobalt, aluminum and nickel, with the balance water.

3. An aqueous solution for stripping nickel consisting essentially of in percent by weight 35-60 HNO 30-50 H SO 0.5-6 of a metal nitrate, the metal of said metal nitrate being selected from the group consisting of silver, magnesium, cobalt, aluminum and nickel, with the balance water.

4. An aqueous solution for stripping nickel consisting essentially of in percent by weight 23-28 HNO 45-50 H 80 0.5-2.4 of a nitrate of silver; with the balance water.

5. An aqueous solution for stripping nickel consisting essentially of in percent by weight 23-28 HNO 45-50 H 80 2-6 of a nitrate of magnesium; with the balance water.

6. An aqueous solution for stripping nickel consisting essentially of in percent by weight 23-28 HNO 45-50 H 50 2-6 of a nitrate of cobalt; with the balance water.

7. An aqueous solution for stripping nickel consisting- References Cited in the file of this patent UNITED STATES PATENTS Hempel Aug. 16, 1938 Lamprey Dec. 2, 1941 OTHER REFERENCES Methods of Stripping Plated Coatings (Brenner), Metal Cleaning and Finishing, November 1933, pages 464-5. 

1. AN AQUEOUS SOLUTION FOR STRIPPING NICKEL CONSISTING ESSENTIALLY OF IN PERCENT BY WEIGHT 20-70 HNO3; AT LEAST 1 H2SO4; AT LEAST 0.5 OF A METAL NITRATE, THE METAL OF SAID METAL NITRATE BEING SELECTED FROM THE GROUP CONSISTING OF SILVER, MAGNESIUM, COBALT, ALUMINUM AND NICKEL; WITH THE BALANCE WATER. 